1. Field of the Invention
This invention relates to a process for the treatment of wastewater, and more particularly to a process for the control of aerating units in a wastewater treatment plant.
2. Description of the Related Art
Wastewater treatment aeration units are commonly controlled by monitoring the oxygen concentration in the wastewater. However, this method of controlling the aeration units has major disadvantages. Oxygen concentration, according to experience, cannot be measured with any degree of accuracy below a level of 0.5 mg/l. Therefore, the lower limit control point for starting operation of the aeration units is set at no lower than 0.5 mg/l of dissolved oxygen. Normally, the aeration units are re-started when the oxygen value goes below the lower control point, even though additional oxygen may not be required because the wastewater treatment process may, under certain conditions, be effectively operated with oxygen concentrations below 0.5 mg/l. Control of the aeration units by monitoring the oxygen concentration thereby results in unnecessary consumption of power and higher than necessary operating costs.
Recently, biological clarification processes have been suggested whereby oxic, e.g., aerobic and anoxic, e.g., anaerobic phases of the wastewater are alternate as, for instance, in the simultaneous denitrification process. In this process, from one-half hour to six hours is normally required to meet the anoxic demands, that is the condition of lowest dissolved oxygen concentration, because of swings in the wastewater conditions, e.g., variations in the oxygen concentration of entering wastewater. Even as much as twelve hours have been found to be required. Anoxic demands cannot be met by controlling the aeration units in accordance with the oxygen concentration since, during the anoxic phase, the dissolved oxygen concentration should be maintained below about 0.5 mg/l and, at a minimum, the oxygen concentration does not fall below the 0.5 mg/l due to the monitoring limitations of the oxygen sensor previously discussed. Hence, the control of the aeration units by the oxygen parameter is not effective in attaining optimum economic operation and cannot be utilized in certain types of further wastewater treatment and clarification, such as in simultaneous denitrification.
Another system for the control of the aeration units constantly measures the turbidity of a clarified effluent from a waste water-activated sludge mixture by means of an electric turbidity meter. Signals representative of turbidity level are utilized to control the aerating units. In this process the aerating units are shut down when a predetermined minimum turbidity value has been reached and re-started only when a maximum value is exceeded. Operating the aeration units in this manner eliminates the above-mentioned disadvantages of controlling the aeration devices by the oxygen parameter, since it conserves energy, reduces operating costs and makes the anoxic denitrification process possible. However, while aeration units are in operation, oxygen concentrations may be attained far in excess of those required by the process. Oxygen concentrations as high as 3 to 5.0 mg/l may be attained in the aeration tank during the oxic phase, even though concentrations of 0.5 to 1.5 mg/l are often sufficient. These uneconomically high oxygen concentrations may be attained if the aerating basins are not fully loaded or if the aeration units are designed for short term, high loadings. Designers, as well as operators of the wastewater treatment plants, encourage the uneconomically high oxygen concentrations if they permit the installation of aeration units having excess capacity, e.g., for safety reasons.
It is therefore an object of the instant invention to provide a method for the treatment of wastewater which economically utilizes the aerating units while providing only that amount of dissolved oxygen required to carry out the wastewater treatment.
It is a further object of the invention to provide a method for the treatment of wastewater which controls the dissolved oxygen concentrations within required levels during both oxic and anoxic phases.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.